Dendrimers are repeatedly branched chemical structures. A review by S. M. Grayson and J. M. J. Frechet, Chem. Rev. 2001, 101, 3819-3867 defines conventional dendrimers as “highly ordered, regularly branched, globular macromolecules prepared by a stepwise iterative approach”. H. Frauenrath, Prog. Polym. Sci. 2005, 30, 325-384 contains the following definition of conventionally prepared dendrimers: “Dendrimers comprise a monodisperse, low molecular weight, multifunctional core unit to which a defined number of dendrons are attached, i.e. monodisperse, branched, oligomeric segments consisting of repeating units with an ABm type functional group pattern and a degree of branching of 100%”.
The first dendrimers were synthesized about 30 years ago by divergent growth, namely by initiating growth at what becomes the core of the polymer and repeatedly coupling and activating to sequentially branch outwards. The convergent approach developed subsequently initiates growth from what eventually becomes the exterior of the molecule. The concept of a “dendron” is relevant in the convergent approach. This can be clearly understood from the above-mentioned reviews by Frauenrath and Grayson (and elsewhere) which explain that a dendron is a “wedge-shaped dendritic fragment”. In the convergent approach, dendrons are synthesized so that they have a functional group at their focal point. Such functional groups are also known as chemically addressable groups and several dendrons (for example three dendrons) are joined together at their focal points and together become the centre of the resultant dendrimer.
By way of analogy, the divergent approach resembles the natural growth of branches on a tree and the convergent approach resembles the preparation of separate branches (dendrons) followed by coupling of the branches.
Dendritic materials have numerous current and potential uses in the chemical, life science, biotechnology and nanotechnology fields as described (for example) in the above-mentioned reviews by Grayson and Frechet and by Frauenrath and also in F. Aulenta, W. Hayes and S. Rannard, European Polymer Journal 2003, 39, 1741-1771 and E. R. Gillies and J. M. J. Frechet, Drug Discovery Today 2005, 10, 1, 35-43. Applications include (for example) delivery devices including drug delivery systems, nanoscopic container molecules, conjugate delivery systems, boron neutron capture therapy, molecular recognition, nanoscopic building blocks, nanoparticles, functionalized or functionalizable materials, optoelectronic uses, single molecule reactions and surface patterning. Some applications exploit the large number of groups of controllable chemistry on the surface of the molecule. Other uses exploit differential properties (for example hydrophobicity and hydrophilicity) between the inside and outside of the dendrimer.
Historically the synthesis of branched polymers with controlled architecture, functionality and size has been demonstrated by the production of so-called ideal dendrimers. However, the production of ideally branched materials requires lengthy procedures and multiple repeated steps of synthesis, purification and characterization. The benefits of the synthesis are often shown in the number and control of the placement of functional groups at the periphery of the molecule. However the synthesis of ideal, regular dendrimer structures is arduous.
Divergent syntheses of dendrimers suffer from the need to react increasing numbers of surface functional groups on each growing molecule to form the next generation of the polymer. Maintaining 100% reaction of all of the available surface groups at each stage of growth is essential for perfect branching and therefore generates complexity within the experimental procedures. Convergent syntheses may overcome the difficulty of exponentially increasing numbers of reactions for each generation by limiting to the coupling of two (or more) wedges. However as the wedges increase in size, steric factors hinder coupling and often make it unsuccessful.
Neither divergent nor convergent methods are practically acceptable for achieving molecular sizes greater than 10 nm (especially in significant quantities).
In the preparation of a vinyl copolymer, L. A. Connal, R. Vestberg, C. J. Hawker and G. G. Qiao, Macromolecules 2007, 40, 7855-7863 disclose a first step of polymerization of a monofunctional vinyl unit (styrene) followed by isolation and purification. This is followed by a separate step of polymerization of a difunctional vinyl unit (divinyl benzene) which necessarily causes gelling and cross-linking.